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Large-eddy simulations of sea breezes over a mountainous island

Abstract

Mountains and coastlines both favor the development of thermally-driven diurnal circulations, including slope flows induced by elevated heating and land/sea breezes generated by differential heating of land and water bodies. Although slope flows and sea-breeze circulations have each been studied intensively using observations and numerical simulations, the interactions between the two remain only partially understood. In particular, some studies suggest an enhancement of the sea breeze in the presence of coastal orography while others point to a blocking or weakening effect. To gain insight into these interactions, we conduct a series of large-eddy simulations of daytime airflow over an idealized Gaussian-shaped island terrain. We analyze the simulated sea-breeze propagation characteristics and frontal-circulation strength under varying environmental and topographic factors, including island geometry (width, terrain height and steepness), boundary-layer static stability, and ambient winds. The results suggest that inland orography accelerates the sea-breeze front inland but also causes a weakening of the baroclinicity and frontal circulation. Over sufficiently tall mountains, the latter effect causes the sea-breeze front to vanish entirely as it ascends the slope. The mountain effects on the sea breeze are quantified by tracking the frontogenesis terms and along-front vertical motion in a sea-breeze-following reference frame. This analysis suggests that the mountain upslope flow acts much like an onshore ambient wind: it hastens the inland frontal propagation but also induces strong frontolysis, both of which scale with the mountain height. The potential for sea-breeze blocking at the foot of the mountain is also assessed through a simple Boussinesq scaling.